In nonferrous smelting, various smelting intermediates are produced, and there exist various forms of substances capable of being smelting materials. Such smelting intermediates and smelting materials may contain valuable metals, but on the other hand, contain arsenic or the like elements unfavorable for environments. For treatment of arsenic, proposed is a method of fixing arsenic in a solution as an arsenic compound by combining arsenous acid with iron, calcium or the like.
The arsenic compound collected as a precipitate is stored or discarded, and it is important that the compound releases little arsenic. As an arsenic compound releasing little arsenic, known is scorodite (FeAsO4.2H2O). However, it is not easy to produce a scorodite crystal as a low bulky form of good filterability, and it has been said that an industrial-scale arsenic treatment process of producing a scorodite crystal could hardly be realized.
The present applicant has succeeded in developing a wet-type process of producing a scorodite-type iron-arsenic compound of good crystallinity as a form thereof of good filterability (Patent Reference 2). The iron-arsenic compound contains arsenic in an extremely high grade of 30% by weight or so, in which arsenic is fixed in the compound and is hardly released out.
Using the wet process, it is possible to industrially produce a crystalline iron-arsenic compound suitable for disposal and storage. However, for its industrial operation, incorrect operation in handling in the production step or the washing step for the iron-arsenic compound must be taken into consideration. For example, in case where the time of iron-arsenic reaction is short or where the amount of washing water is too small or where the washing method is incomplete, then it may be considered that the amount of arsenic adhering to the iron-arsenic compound may increase more than usual. In case where the environment of a discarded, accumulated or stored iron-arsenic compound has changed to an alkali side, then arsenic release from the scorodite crystal may increase.
In consideration of the current situation as above, the present applicant has proposed an arsenic-containing solid in which a scorodite-type iron-arsenic compound of good crystallinity as above and an iron oxide compound (iron oxide or iron oxyhydroxide) coexist therein (Patent Reference 3). This provides a stable arsenic release retardation effect in the release test according to the Japan Act (Notification No. 13 by the Ministry of the Environment in Japan), and even in pH change in different disposal, deposition or storage environments, arsenic release can be still kept low.